Coaxial line oscillator



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Patented Mar. 31,1953 *C* 1 COAXIAL LINE OSCILLATOR William R. Rambo,Cambridge, Mass., assignor to the United States of America asrepresented by the Secretary of War Application December 10, 1945,Serial No. 634,097

(Cl. Z50- 36) 4 Claims. 1

,same time as a frequency tripler.

The upper frequency limit at Which a conventional triode vacuum tubewill oscillate at its fundamental frequency is limited by the physicalstructure of the vacuum tube, the transit times of the electrons betweenthe cathode and the plate, and increases in power loss throughstructural materials and dielectrics at the higher frequencies.Accordingly it is an object of this invention to extract radio frequencyenergy from a conventional triode Vacuum tube operatingas an oscillatorat frequencies beyond its normal Working range.

The above object will be satised if the plate tank circuit of theoscillator is resonant at the fundamental frequency and at the same timeat the third harmonic of the frequency of oscillation. It is a furtherobject to provide a device of the required function cited above in whichthe tuning may be made not too critical.

Other objects, features and advantages of this invention will suggestthemselves to those skilled in the art and will become apparent from thefollowing description of the invention taken in connection with theaccompanying drawing in which:

Fig. 1` is a cross sectional view of an oscillator-tripler circuit usinga conventional triode vacuum tube and resonant coaxial line tankcircuits; and

Fig. 2 is a schematic diagram of the oscillatortripler circuit shown inFig. 1.

Referring more specically to Fig. 1 there is shown therein a pictorialView of a conventional triode vacuum tube I which may be of thelighthouse type. Triode I0 includes a cathode II, control, grid I2 andanode pillar I3, each separated by a suitable insulating material I4Center conductor 20 of a coaxial line associated with the triode Ill isconnected to outer shell 42 of triode I0 which in turn is capacitivelycoupled to cathode I| through a small thickness of dielectric material43. Outer conductor 2| of the aforesaid coaxial line is joined tocontrol grid I2 through metal disk 22. Center conductor 20 and coaxialouter conductor 2| form a resonant coaxial line between the cathode andthe control grid of triode I0. Cathode II is connected to centerconductor 20 2 through bias resistor 44. The terminating en of thegrid-cathode resonant cavity is short-circuited by means of anadjustable annular piston 23.

One end of center conductor 30 of another coaxial structure is connectedto anode pillar I3 and the opposite end is connected to a suitablesource of positive potential, designated herein as B+.

One end of coaxial conductor 3|, the diameter of which is greater thanthat of center conductor 30, contacts center conductor 30 by means ofsliding contact 32. The opposite end of coaxial conductor 3| isconnected to an insulating collar 33 surrounding center conductor 30.Insulating collar 33 contains a screw 36 which projects vthrough slot 34cut in a second coaxial conductor 35. 'Ihe diameter of coaxial conductor35 is greater than the diameter of coaxial conductor 3|. Adjusting nut3l on screw 36 allows the insulating collar 33 and coaxial conductor 3|assembly to be adjusted relative to center conductor 30.

One end of coaxial conductor 35 is joined to a metal disk 38 which inturn is screw-connected to metal disk 22. A second insulating collar I1surrounding center conductor 30 is attached to the opposite end ofcoaxial conductor 35.

The electrical terminating end of coaxial conductor 35 isshort-circuited for radio frequencies to the electrical terminating endof coaxial conductor 3| by means of an annular piston I5, surroundingcoaxial conductor 3|. The sliding member of annular piston I5 whichcontacts coaxial conductor 3| is separated from the sliding member whichcontacts coaxial conductor 35 by a piece of dielectric material I6. Thisprevents the plate potential from being shorted to the control grid ofthe triode vacuum tube and allows the radio frequency potential to bebypassed. The position of the annular piston I5 is adjustable by meansof a rod such as 40 attached thereto and projecting through insulatingcollars 33 and I1 to the outside. Center conductor 30, coaxial conductor3| and coaxial conductor 35 form a resonant coaxial line between thecontrol grid and anode pillar of the triode.

Feedback between the grid-plate and the gridcathode resonant coaxiallines is provided by a small feedback probe 45 projecting through a holecontaining an insulating material 4I in disks 22 and 38.

Power is extracted by a pickup probe 46 which projects through a hole I8in coaxial conductor 35 and is applied through a high pass filter 41 tothe apparatus which utilizes the output of the illustrated device.

For a more detailedexplanation of the operation of this circuit,reference will now be made to Fig. 2 which is an equivalent schematicdiagram' of the device shown in Fig. 1. All elements designated in Figs.1 and 2 by the same reference numbers are identical in function andpurpose. In this oscillator circuit the vtube gridplate capacitance istuned by the inductive irnpedance of the anode pillar and the twolengths of the coaxial lines, all coacting in series. The inductiveimpedance formed by the anode pillar within the tube envelope is shownas Z1 in Fig. 2. The inductive impedance formed by the first section ofcoaxial line, Z2, also shown in Fig. 2, is adjusted through motion ofthe sliding contacts at 32. The inductive impedance formed by the secondsection of coaxial line, Za, also shown in Fig. 2, is adjusted throughmotion of the annular piston I relative to the sliding contact at 32.

. The characteristic impedance of the two scctions of coaxial linesdiffer from one another since each center conductor is of a differentdiameter. For a given set of characteristic impedances, a combination ofline lengths can be found that will resonate the resonant coaxial linesat both the fundamental and at the third harmonic frequency ofoscillation. Viewed from the anode pillar of the vacuum tube, theeffective impedance offered to the fundamental frequency must be threetimes that offered to the third harmonic frequency in order that theabove resonant conditions will exist. The electrical length of thegrid-anode resonant cavity is there.. by dependent upon the frequencyapplied to it.

The cathode-grid resonant coaxial line is a single-characteristicimpedance coaxial line, end loaded with the tube capacitance. It istherefore impossible that it be resonant at both the fundamentalfrequencies and at three times the fundamental frequency. The annularpiston in the grid-cathode resonant coaxial line is adjusted so that itwill be resonant at the fundamental frequency of oscillation.

The feedback is designed to operate efficiently at the fundamentalfrequency in developing a driving voltage across the resonantcathode-grid impedance. There is therefore no significant third harmonicfeedback into the grid-cathode resonant coaxial line. The tubeoscillates as an oscillator at a frequency 1A; that of the desiredoutput frequency.

While there is a wide range of line lengths and characteristicimpedances that can satisfy the tuning requirements insofar as thefundamental is concerned, simultaneous tuning to the third harmonic issomewhat critical. This sharpness of tuning is accentuated as theterminating capacitance of the vacuum tube is increased or, for a xedcapacitance, as the frequency is increased. For a given capacitance andfrequency, tuning is least critical when the characteristic impedancesof the two lengths of concentric lines, Z2 and Z3, are widely separatedin value.

Power is capacitively coupled from the gridplate resonant circuit by theprobe 46 which is placed in the line at a point where the third harmonicvoltage signal is at a maximum, this being at a distance M4 (at thethird harmonic) from annular piston I5 as shown in Fig. 2. The probe 46feeds the load through a T-section high-pass filter that attenuates thefundamental and passes the third harmonic, This limits the 94 output toenergy of the desired high frequency.

Since the oscillator is essentially underloaded on the fundamentalfrequency (except by the driving impedance), high amplitude alternatingvoltages exist that provide suicient excitation and also bias acrossbias resistor 44 and capacitance coupling 43 to permit shortening theelectrical angle of plate current now to values conducive to thegeneration of harmonics.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as set forth nthe appended claims.

The invention claimed is:

1. In combination, an electron discharge device having at least acathode and a control grid and an anode, a first resonant coaxial lineconnected between said control grid and said cathode, a means foradjusting the physical length of said first resonant coaxial line sothat it will be resonant at a particular radio frequency, second andthird resonant coaxial lines of different radial dimensions and coactingin series to form a fourth resonant coaxial line between said controlgrid and said anode, means for independently adjusting the physicallengths of said second and third resonant coaxial lines so that saidfourth resonant coaxial line is simultaneously resonant at both thefundamental and the third harmonic of said radio frequency, and meansfor extracting radio frequency energy of substantially said thirdharmonic frequency from said fourth resonant coaxial line.

2. An ultra-high frequency oscillator comprising an electron dischargedevice having a cathode and a grid and an anode, a first resonantcoaxial line coupled to said grid and cathode and tuned to a given radiofrequency, a second resonant coaxial line coupled to said grid andanode, said second line including a pair of coaxial lines of differentradial dimensions connected in series, means for independently adjustingthe length of said last-mentioned Icoaxial lines so that said secondcoaxial line simultaneously tunes the grid-to-plate interelectrodecapacitance of said tube to said given radio frequency and to a thirdharmonic thereof, coupling means in said second line at a point whereenergy of said third harmonic is a maximum`and energy of said givenfrequency is substantially less than that of said third harmonic forproviding output energy of said third harmonic, and means coupling saidfirst line to a point in said second line at which energy of said givenradio frequency is a maximum and energy of saidthird harmonic issubstantially less than that of said given radio frequency for supplyingfeedback energy of said radio frequency to said first line.

3. An ultra-high frequency oscillator comprising an electron-dischargedevice having at least a cathode and a control -grid and an anode, afirst resonant coaxial line connected between said control grid and saidcathode, means for adjusting the length of said rst coaxial line so thatit will be resonant at a given radio frequency, a second coaxial linewith a given spacing between conductors coupled at one end to.

said grid and said anode, a third coaxial line with a given spacingbetween conductors smaller than that of said second coaxial line andcoupled at one end to the other end of said second line andshort-circuited at its other end, means:

for independently adjusting the lengths of said second and'y third linesso that the coaxial line formed by said second and third lines issimultaneously resonant at said given radio frequency and at the thirdharmonic of said radio frequency, means in said third line atsubstantially a quarter of a Wavelength of said third harmonic from saidshort-circuited end for obtaining output signals of said third harmonic,and means coupled between said first line and a point in said secondline at which energy of said given radio frequency is a maximum andenergy of said third harmonic is substantially less than that of saidgiven radio frequency for providing feedback energy of said given radiofrequency to said first 15 2,476,725

coaxial line.

4. An oscillator according to claim 3 in which said output couplingmeans includes a high pass filter for attenuating said given radiofrequency and for passing said third harmonic.

WILLIAM R. RAMBO.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,379,952 Heedene July 10, 19452,408,355 Turner Sept. 24, 1946 Gurewtsch July 19, 1949

